linux-kernel - Re: Question regarding mutex locking

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Message-ID: <474D8C22.7010902@lwfinger.net>
Date:	Wed, 28 Nov 2007 09:41:22 -0600
From:	Larry Finger <larry.finger@...inger.net>
To:	Andreas Schwab <schwab@...e.de>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question regarding mutex locking

Andreas Schwab wrote:
> Larry Finger <Larry.Finger@...inger.net> writes:
> 
>> If a particular routine needs to lock a mutex, but it may be entered with that mutex already locked,
>> would the following code be SMP safe?
>>
>> hold_lock = mutex_trylock()
>>
>> ...
>>
>> if (hold_lock)
>> 	mutex_unlock()
> 
> When two CPUs may enter the critical region at the same time, what is
> the point of the mutex?  Also, the first CPU may unlock the mutex while
> the second one is still inside the critical region.

Thank you for that answer. I think that I'm finally beginning to understand.

Larry
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